The present invention relates to devices for monitoring and controlling the flow of air and other gaseous fluids in ducts or conduits, and specifically to a modular control device for detecting duct velocity in a laboratory fume hood and for adjusting the velocity to maintain specified values.
Fume hoods are provided in laboratories for removing noxious or toxic fumes during the running of experiments or tests which utilize or generate such fumes. Typically, the fume hood includes an enclosure with doors dimensioned so that experiments may be carried out within the confines of the fume hood. Exhaust ducting is provided to expel the air and any noxious fumes so that the laboratory technician will not be exposed to them while working near the hood.
Fume hood controllers control the flow of air through the fume hood as a function of the desired average face velocity of the effective opening of the fume hood. The average face velocity is generally defined as the flow of air into the fume hood per square foot of open face area of the fume hood, with the size of the open face area being dependent upon the position of one or more of the moveable doors that are provided on the front of the fume hood.
Fume hoods can be exhausted by an exhaust system including one or more blowers capable of being driven at variable speeds to increase or decrease the flow of air from the fume hood, which compensates for the varying size of the opening or face. Alternatively, there may be a single blower connected to the exhaust manifold that is in turn connected to the individual ducts of multiple fume hoods. Dampers are provided in the individual ducts to control flow therein, and to thereby modulate the flow and maintain the desired average face velocity. There may also be a combination of both of the above-described systems.
The doors of many fume hoods can be opened by raising them vertically to what is often referred to as the sash position. Alternatively, some fume hoods have a number of doors that are mounted for sliding movement in typically two sets of tracks. There are even doors that can be moved horizontally and vertically, with the tracks being mounted in a frame assembly that is vertically moveable.
In conventional damper controlled applications, fume hood duct velocity is largely a function of the damper position, and is detected by installing at least one so-called Pitot tube in the center of the duct. Each Pitot tube includes a first tube with a total pressure sensing orifice, and a second tube having a static pressure sensing orifice. The velocity pressure of air flow in the duct is calculated from the difference of the total pressure and the static pressure measured at the orifices. However, the centralized and exposed location of the Pitot tube causes it to be easily plugged with dust or other foreign matter, and thus accuracy is impaired.
Another disadvantage of conventional fume hood control systems is that the pressure differential monitoring equipment, and the damper mechanism which is used to vary the flow of air within the exhaust duct, must be installed on site during assembly of the fume hood. This procedure, which must often be performed under cramped conditions, is labor intensive and may be awkward and uncomfortable to the installer.
Thus, a first object of the present invention is to provide an improved laboratory fume hood terminal wherein the Pitot tube or other pressure differential sensing devices are disposed to avoid clogging.
Another object of the present invention is to provide an improved modular fume hood exhaust terminal which may be installed as a unit into a fume hood exhaust system with substantially much less on site labor required than in conventional installations.